1. Field of the Invention
The present invention relates to a controller for an electric power steering system for providing a steering assist force by a motor to steering systems of an automobile or a vehicle, particularly to a controller for an electric power steering system for automatically correcting an offset of a motor-current detection system and balancing the steering assist force so as to improve the steering feeling.
2. Description of the Prior Art
An electric power steering system for auxiliary-load-energizing the steering system of an automobile or a vehicle with the rotating force of a motor is designed to energize an assist force to a steering shaft or a rack shaft by a transmission mechanism such as a gear or a belt through a decelerator. This conventional electric power steering system performs feedback control of a motor current in order to accurately generate an assist torque(steering auxiliary torque). The feedback control adjusts a motor-applied voltage so that the difference between a current control value and a motor current detection value decreases, and the motor-applied voltage is generally adjusted by adjusting the duty ratio of PWM(pulse-width modulation) control.
Then, a general configuration of an electric power steering system is described below by referring to FIG. 1. A shaft 2 of a steering handle 1 is connected to tie rods 6 of a steering wheel through a deceleration gear 3, a universal joints 4a and 4b and a pinion rack mechanism 5. The shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering handle 1 and a motor 20 for auxiliary of the steering force of the steering handle 1 is connected to the shaft 2 through the deceleration gear 3. Power is supplied to a control unit 30 for controlling the power steering system from a battery 14 through an ignition key 11 and a relay 13, and the control unit 30 computes a steering assist command value I of an assist command in accordance with a steering torque T detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12 and controls the current to be supplied to the motor 20 in accordance with the computed steering assist command value I.
An electric power steering system using a DC motor realizes an optimum steering force by controlling a motor current. However, because a motor current detection circuit is necessary to control a motor current, such a motor current detection circuit is generally constituted by an analog circuit. The analog circuit latently has a DC offset. Particularly, in the case of a current detection circuit for electric power steering, it is necessary to detect a large current of tens of amperes at a low voltage of 12[V] and a high resolution. Therefore, it is necessary to amplify a signal having a small potential difference up to the maximum value of a detection range. Therefore, when using the above high-gain amplifier, a problem occurs that a latent offset is also amplified and increased. When the above offset is present, a problem occurs that a current seems to flow though the current does not actually flow and thereby, a preferable assist cannot be performed.
The above offset value is conventionally set so that the characteristic of a current detection circuit does not influence the maximum value of offset values which may be generated and the whole system is not influenced even if an offset is generated.
FIG. 2 shows a configuration of the control unit 30 having an offset correcting function, which is divided into a realizing portion by software and a realizing portion by hardware. The steering torque T detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12 are inputted to a current command computing portion 31. The current command computing portion 31 computes and decides the steering assist command value I which is the control target value of a current to be supplied to the motor 20. The steering assist command value I is inputted to a subtracter 32 and a current control value E which is a subtraction result by the subtracter 32 is inputted to a motor driving portion 34 through a driving control means 33, and the motor 20 is driven by the motor driving portion 34. The current of the motor driving portion 34 is detected by a current detecting portion 35 and a current detection signal is inputted to a subtracter 36. Moreover, an offset value from an offset setting portion 37 according to a measurement result is inputted to an offset correcting portion 38, a correction signal from the offset correcting portion 38 is inputted to the subtracter 36, and a corrected current detection signal from the subtracter 36 is inputted to the subtracter 32.
Moreover, to correct an offset by hardware, the circuit configuration shown in FIG. 3 is set to a state in which an output of the output-driving circuit 34A of a CPU 30A is turned off, that is, a current(A) circulating through the motor 20 becomes “0” and the then output voltage(V) of a current detecting circuit 35A is measured to adjust an offset correcting circuit 38A so that the voltage(V) becomes “0”.
However, in the case of the method where the characteristic of a current detecting circuit is set so that it does not influence the maximum value of offset values which may be generated, there is no problem when an offset which may be generated to a resolution necessary for control is small enough. However, when a large offset is generated, problems occur that a necessary resolution is not obtained but a scale much larger than a scale necessary for control must be set and only a performance far behind a performance which can be originally realized to correct an offset can be realized.
Moreover, as shown in FIG. 2, a system provided with components(the current detecting portion 35, the offset setting portion 37 and the offset-correcting portion 38) capable of previously correcting an offset by the hardware has problems that an extra circuit is necessary to adjust the system, it takes a lot of time to adjust the system, and the manufacturing cost is increased in any case. Furthermore, to realize an electric power-steering system having a higher performance, it is necessary to further raise the resolution of a current detecting circuit and thereby, the resolution of offset correcting means must be raised, and thus, it is necessary to more severely adjust the offset correcting means.
That is, because an offset is conventionally corrected by hardware, a circuit for correction is necessary, excessive component cost and mounting area are necessary and moreover, man-hours for the correction are necessary and therefore, the cost for adjusting the circuit is increased. Furthermore, there is a problem that when using a variable resistor(trimmer) in order to realize a correcting circuit, an adjustment error becomes small but a positional shift after adjustment may occur. On the other hand, when using an adjusting resistor(selection of fixed resistor) in order to realize a correcting circuit, there is a problem that an adjustment error becomes large though no shift occurs after adjustment. Because of selection from a commercial fixed resistor in variation, there is a disadvantage that it is impossible to adjust a value smaller than the difference width between commercial resistors.